Field of the Invention
The present invention relates to a bulk acoustic wave (BAW) device which uses a bulk acoustic wave that is propagated through a substance, and a method of manufacturing such a BAW device.
Description of the Related Art
Bandpass filters for passing only electric signals in desired frequency bands play an important role in radio communication devices including mobile phones. As one of the bandpass filters, there has been known a surface acoustic wave (SAW) device (SAW filter) which uses surface acoustic waves. The SAW device includes, for example, a crystal substrate made of a piezoelectric material such as crystal (SiO2) and interdigital electrodes (IDT: interdigital transducer) formed on the face side of the crystal substrate. The SAW device passes only an electric signal in a frequency band determined depending on the type of the piezoelectric material, the gap between the electrodes, etc.
The SAW device may find itself in a situation where part of an elastic wave generated in the vicinity of an input electrode is propagated through the crystal substrate and reflected by the reverse side of the crystal substrate. When the reflected elastic wave reaches an output electrode, the frequency characteristics of the SAW device are deteriorated. One solution has been to form an uneven microstructure on the reverse side of the crystal substrate for making it easy to scatter the elastic wave, thereby preventing the reflected elastic wave from reaching the output electrode (see, for example, Japanese Patent Laid-Open No. 2003-8396).